in the early days you had to buy salt in bulk and it was not always available. When you got it, you had to protect it from moisture. So it was kept in a "saltbox" on a shelf above the kitchen stove.

Of course you mean table salt, Dr. Bill ? My Grandmother had a salt box where she stored table salt even when it was easy to purchase in stores ... a holdover from earlier times.
I've never seen a salt box to store "melting" salt. In nearby Portsmouth, on the river's edge, there are huge piles of salt - out in the open - for spreading on icy roads.